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Last week, I pointed out some of the pitfalls inherent in clustering (see the first URL below). I mentioned that clustering Exchange Server can't mitigate the biggest single point of failure: administrator error. A poorly designed or administered cluster will reduce uptime instead of increasing it. With that fact in mind, let's talk about design principles that can help you build the best cluster for your buck.

First, remember that clustering isn't a silver bullet. Let's say you put Exchange on a two-node cluster, then lose power to your server room. Only then do you find out that your UPS is bad. The cluster can't help you now. (Even if your UPS is good, you'll be in the same boat if you don't have a way to cleanly shut down the cluster before the UPS power runs out.) Clustering can't entirely protect you from infrastructure problems such as electrical or cooling problems, fire, or flooding. Also, you can't use clustering for every Exchange 2000 Server service. (See the second URL below for a Microsoft article that lists which services you can and can't cluster.)

Second, think carefully about why you want a cluster. If you're trying to reduce unplanned downtime, failover times will be of paramount importance, and the biggest determiner of failover times is how many log files the system must play back. Frequently perform full backups to keep your log file count low--and don't turn on circular logging. If you're more interested in providing uninterrupted service during times of planned maintenance or providing transparent service to users, failover times might not be as important.

Third, bear in mind that clustering can't repeal the laws of nature--or of the Exchange engineering team. One Exchange 2000 server can mount a maximum of four storage groups (SGs). If you have two active/active nodes, each with three SGs, the cluster won't mount two of those SGs when a failover occurs. Oops! Don't put more than two SGs on each cluster node.

Speaking of storage: Because the storage subsystem is the only component whose failure can bring down both nodes in a cluster, make sure you pay careful attention to properly selecting and provisioning that subsystem. Keep your SG log files and databases on separate volumes, and use whatever monitoring tools the storage vendor provides to keep an eye out for incipient failures so that you can fix them before you lose data.

Last, listen to Ed Heinemann, a famous aeronautical engineer and aircraft designer from the 1930s to the 1970s. Heinemann's motto became well-known in the aviation community: "Simplicate and add lightness." It might make an English teacher cringe, but engineers--and probably most systems and network administrators--understand what the man meant: Unnecessary complexity is the enemy of reliability and performance, so simplifying the design is crucial.

The bottom line: If you can clearly articulate why you need clusters, can justify their cost (including care-and-feeding costs such as maintenance agreements and administrator training), and can specify a design that will provide the level of service you need--use clusters. If you can't do these three things--don't use them.

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* HOW TO: RESTORE AN INFORMATION STORE DATABASE IN A CLUSTERED EXCHANGE ENVIRONMENT Each week, Microsoft posts several Exchange Server how-to articles to its Knowledge Base. This week, learn how to restore an Exchange Server Information Store (IS) database on a cluster server. http://support.microsoft.com/?kbid=303949

* FEATURED THREAD: OUTLOOK TO EXCHANGE SYNC HANGS OVER VPN Bazz_OZ has a problem synchronizing Outlook to an Exchange 2000 Server through a VPN. To offer your advice or join the discussion, go to the following URL: http://www.winnetmag.com/forums/rd.cfm?cid=40&tid=54350

* BACK UP AND RESTORE EMAIL MESSAGES LIUtilities released WinBackup, software that lets you organize and schedule automatic backups. WinBackup uses encryption techniques to protect your data, compresses data to save space, and creates detailed logs of all backup operations. One feature backs up your Outlook messages and can locate and back up your address book. You can also use WinBackup to restore email messages: Simply select the email item on the restore page and WinBackup will find the email folder and restore your email messages. WinBackup can also restore your address book. Pricing ranges from $29.95 for a CD-ROM to $49.95 for the boxed version. Contact LIUtilities at [email protected] http://www.liutilities.com

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